Study On Failure Mechanism Of High Capacity Electrode Materials For Lithium Batteries Considering Strain Gradient Plasticity Theory

Lithium battery has been widely applied in mobile communications,traffic transportations,electrical energy storages,aerospace industries and many other fields because of its numerous outstanding properties,such as the high voltage platform,high energy density,long cycling life,low self-discharge rate,zero memory effect and environmental protection,etc.With the rapid development of science and technology,the energy densities of commercial lithium battery electrode materials are approaching their theoretical limits.In order to meet the increasing demand of people,it is urgent to develop and produce high capacity lithium battery electrode materials.However,many high-capacity electrode materials cause serious volume deformation and structural destruction during the first cycle of charging and discharging,resulting in a large number of irreversible capacity loss,and even causing safety problems such as combustion or explosion.For reducing the excessive volume deformation,the current common method is to make electrode materials into some structures of micro or nano scale.In recent years,it has been found that many high capacity electrode materials of micro or nano scale have the scale effect of mechanical response in the process of charging and discharging.A phase transition reaction takes place in the front of lithium diffusion forming a high-density dislocation area,in which there is a distinct strain gradient.Therefore,in this thesis,the strain gradient plasticity?SGP?theory is introduced and the finite element calculation simulation software ABAQUS and user subroutine are applied to establish the constitutive relationship of the high-capacity thin-film electrode materials in lithium batteries considering diffusion and strain gradient plasticity theory.The intention here is to analyze the effect of strain gradient plasticity on the structural shape and stress evolution of lithium electrode materials.The main contents are as follows:?1?The thin film electrode structure model of lithium diffusion is established.The influence of dislocation and strain gradient is introduced into the classical J₂ flow theory.The elastic-plastic deformation equation considering SGP theory is built.The simulation is carried out by using the finite element simulation software ABAQUS and the user subroutine compiled according to SGP theory.Although there is no mature coupled electrochemical-mechanical module in ABAQUS,the coupled thermal stress analysis module of ABAQUS is selected for equivalent analogy because the structure of diffusion equation is similar to that of heat conduction equation.?2?Qualitative analyses on the morphology,concentration evolution,plastic yield and stress dynamic evolution of lithium structure are carried out.It is found that there is a sharp change of Li concentration distribution at the two-phase boundary of the film at the beginning of lithium,which leads to the uneven expansion of the film and serious stress mismatch.Yield occurs preferentially in the lateral region,the edge of the bottom and the central region of the film.The large horizontal tensile stress on the side of the interface is the main cause of the interface damage and debonding,and the vertical tensile stress in the middle axis is the main reason of the horizontal crack initiation in the film.The stress at the middle point of the upper surface develops from compressive stress at the early stage of lithiation to tensile stress at the later stage.The former makes ratcheting phenomenon emerge,while the latter leads to the vertical crack on the upper surface.?3?The effects of the physical parameters related to the debonding of the interface and the fracture of the upper surface on the charging process were studied.It was found that reducing the initial thickness h₀ of the thin film active material can effectively relieve debonding of interface and fracture on upper surface.For the destruction-related physical quantities,reducing the initial cohesive stiffnessK_i⁰ or increasing the contact traction strength?_c⁰ of the contact surface can effectively alleviate damage of interface and damage of upper surface.However,increasing the initial cohesive stiffnessK_i⁰ can only alleviate interface debonding,while reducing contact traction strength?_c⁰ or increasing the composite fracture energy?_c both can better resist the occurrence of interface debonding and fracture on upper surface.